The present invention generally relates to communication control units, and more particularly to a communication control unit for controlling a communication made through a communication network employing a multi-conjunction architecture.
Conventionally, as communication networks applicable to local area networks (LANs), there are a network employing a carrier sense multiple access with collision detection (CSMA/CD) method using coaxial cables for buses as typified by an Ethernet (registered trademark), an optic star shaped network using optical fibers for buses, and an optic loop shaped network employing a time-division multiple access (TDMA) method. The network using optical fibers is less affected by noise caused by external electromagnetic waves when compared to the network using coaxial cables.
In the network using buses such as the Ethernet, a fault in a node apparatus will not cause the entire system to go down, but the entire system will go down when a node apparatus oscillates or a cable breaks. In the case of the star shaped network, the entire system will go down when a fault occurs in a central part of the network. Furthermore, in the case of the loop shaped network, there is a possibility of the entire system going down when a fault occurs in a node apparatus or a link. It is possible to provide the loop in duplicate, but it would make the construction of the node apparatus extremely complex.
Accordingly, a lattice communication network analogous to nerve cells of a living body has been previously proposed in a U.S. Pat. No. 4,516,272. According to this proposed lattice network, each node apparatus has a plurality of input and output channels and is used as a communication control element, and such node apparatuses are coupled in a multi-conjunction to constitute the lattice network. Each node apparatus transfers one of incoming digital signals according to a first-come-first-served logic (hereinafter referred to as a first-come-first-output logic).
The proposed lattice network has a large degree of freedom of network topology because of the multi-conjunction architecture. Thus, the survivability of data is high. In other words, even when a fault occurs in a path of the network, it is possible to carry out the communication through other paths. Furthermore, it is possible to select optimum paths for the communication because each node apparatus transfers the incoming digital signals according to the first-come-first-output logic.
In the lattice communication network in which a communication may continuously occupy in time a part or all of the communication network, a terminal coupled to the communication network must send through the communication network a communication request signal intended for a destination terminal. A terminal which receives the communication request signal must discriminate whether or not the communication request signal is intended therefor, and the terminal must send an acknowledge signal when the received communication request signal is intended therefor. In addition, the terminal which sends the communication request signal and the terminal which receives the communication request signal and sends the acknowledge signal must fix a communication path therebetween in the communication network.
For this reason, each terminal in the lattice communication network requires an interface having a function of recognizing an address, so that the terminal can discriminate whether or not the received communication request signal is intended therefor. However, a standard interface which is commonly available and generally used does not have the concept of address. Hence, there is a problem in that the standard interface cannot be used in the communication network employing the multi-conjunction architecture.
Because the standard interface does not have the concept of address, it is impossible to discriminate whether or not a communication release instruction signal received after a communication path is fixed in the communication network is intended therefor even when an attempt is made to use the standard interface in the communication network employing the multi-conjunction architecture. Consequently, it is impossible to effectively release the fixed communication path after the communication is ended so as to open the communication network for other communications.
On the other hand, when a communication control unit is to be used with a plurality of interfaces, it is necessary to provide a number of communication control units in correspondence with the number of interfaces. As a result, a large space is required to set up the communication control units, and a power source having a large capacity is required. Furthermore, the communication control unit must be designed independently for each kind of interface, and there is a problem in that it takes considerable time and effort to design the communication control units for the various kinds of interfaces.